- Aperture and Blurring.
- Sensor size and Megapixel counts.
The two are related.
Diffraction is an optical effect. Without boring you with physics it means that light that hits an edge will spread out beyond the edge.
Here's a more detailed explanation of the idea.
Aperture first. The aperture in a lens is, in one form or another, simply an opening whose size is adjusted to let in more or less light.
Diffraction becomes more obvious ( is a bigger effect ) as the size of the opening decreases.
The net effect of diffraction is to cause an unavoidable blurring as we increase f-number, because as we increase f-number we decrease the size of the aperture.
For this reason as we raise aperture the ability of the camera to resolve detail diminishes unavoidably. It is physically impossible to overcome this effect. It is for this reason that lenses normally peak in their resolution power at about f8 or f11. It is around this point that the diffraction blurring becomes too strong for the optics to overcome.
It is in the nature of the diffraction process that you cannot undo it using computation techniques. Or put another way you cannot reverse the process.
This is why we avoid very high f-numbers if we can.
However the second effect is very important to.
Sensors are made of millions of sensor elements which are effectively our pixels. These collect light particles ( photons ) as they hit the sensor.
Sensor elements are very small. Diffraction, even at f8 or f5.6 for example, can be so large that the photon is spread by diffraction over more than one sensor element.
Now sensors are built in a few sizes. And as manufacturers increase the number of pixels the area available for those pixels does not increase. So each sensor element is smaller when we increase the number of pixels on the sensor.
The smaller the sensor element the easier it is for diffraction blurring to affect it.
As a result the sensors in point and shoot cameras ( which are very small ) are badly affected by diffraction.
In fact this is so bad that diffraction effects image quality at all apertures on some sensors.
It is also why the larger sensors in DSLRs are able to resolve detail better than a P&S camera, even when the P&S has far more pixels. In fact the larger the pixel count on the P&S, the worse you get.
In a practical sense this means that 6 - 10 Mp is actually as large as you can usefully go on a P&S.
But even then a 6 Mp DSLR will out resolve a 10 Mp P&S.
Physics is a cruel mistress. She shows no mercy and offers no favours. In this case she makes rather a mockery of the increasing megapixel counts on P&S cameras.
So when considering a purchase of a camera choose the one with the largest sensor. The largest sensor has many advantages, and this is one.
